SIP (Session Initiation Protocol) is a standard set as one of call control protocols achieving call control such as connection request with a realtime application of an internet phone based on VoIP (Voice over Internet Protocol). (For example, see Non-Patent Document 1.)
SIP was used originally for VoIP communication between computers such as personal computers and servers connected via a wired network on the internet. In recent years, however, a SIP server and a wireless communication terminal (apparatus) of wireless carriers (operators) supporting SIP has been developed, as digitalization of and adoption of IP technology to a wireless communication equipment has been enhanced. Furthermore, the recent development of wireless communication technology enables a single wireless communication terminal to be provided with a plurality of wireless communication devices connectable to different wireless communication networks. Typical constitutions are PDA, PC or a cellular phone provided with a first wireless communication device to connect to a cellular phone network and a second wireless communication device to connect to a station of a wireless LAN such as WiFi and WiMAX, or MAN (Metropolitan Area Network).    Non-Patent Document 1: the website at http://www.ietf.org/rfc/rfc3261.txt (a document of RFC defining SIP)